1. Field of the Invention
This invention relates generally to a prehensor and, more specifically, to a voluntary close prehensor that sequentially varies the ratio of input control cable tensile force to actual gripping force to obtain energy-efficient mechanical advantage during gripping.
2. Description of the Prior Art
In the United States, approximately 90,000 individuals are considered to be upper-body amputees, having lost all or a portion of an upper limb. Of those, a subset will use a prosthetic (artificial) arm to enhance or restore their capabilities and, hopefully, their quality of life. While a number of prosthetic devices have been developed to assist these individuals, their use is not wide-spread due, at least in part, to the poor performance and design of existing prosthetics and prehensors. A prehensor, also known as a "gripper" or an "end-effector," is a mechanical grasping device used by an upper-body amputee to serve as an artificial hand.
In response to the need for prosthetic arms and associated prehensors, two types of devices have been developed and marketed: battery-powered electronic systems and body-powered mechanical systems. Electronic systems using a battery package, electric motors, and sophisticated electronic controls have been developed and shown to work reasonably well. Unfortunately, electronic systems are very expensive, often costing $35,000 or more for entry models. In addition, electronic systems can be unrealistically heavy and suffer from inadequate battery life. U.S. Pat. No. 4,792,338 issued to Rennerfelt discloses an electronic or battery-powered prehensor.
The second type of prosthetic devices commonly used are called "body-powered" systems" because the wearer controls the system using muscles in his or her body, usually muscles in the shoulder and neck. Body-powered mechanical systems are generally lighter, quieter, and far less expensive than their electronic counterparts, and do not suffer from battery-life limitations.
Within the realm of body-powered prosthetics, there are two primary families, differing primarily in the type of prehensor used. Voluntary open (VO) prehensors typically include two or more gripping digits (mechanical fingers with rubber pads for friction and better grip) that are held or biased against each other by a spring or one or more strong rubber bands. The wearer moves the digits apart prior to gripping by pulling on a control cable connected to the wearer's shoulder and neck through a harness. When the wearer relaxes or eases the tension on the control cable, the digits close on the object to be held and "grip" it. In essence, voluntary open prehensors are spring loaded clamps that can be opened at will by the wearer. Therefore, with a voluntary open prehensor, the wearer's grip on the object is passive and the wearer need do nothing to maintain the grip.
Voluntary open prehensors are popular due to their low cost as compared to electronic prehensors, and the fact that the wearer does not expend energy while gripping an object. Unfortunately, since gripping an object with a voluntary open prehensor is passive, i.e., the wearer is not expending energy to maintain the grip, the wearer has limited, if any, control over the amount of force exerted on the object. Gripping forces needed to lift heavy objects are excessive for small or lightweight fragile objects. Conversely, the correct gripping force needed to lift a light object will usually be inadequate for heavier objects. U.S. Pat. No. 3,604,017 issued to Brown et al. and U.S. Pat. No. 5,116,386 issued to Scribner disclose voluntary open prehensors.
The second major type of body-powered prehensors are the voluntary close (VC) prehensors. As its name implies, unlike a voluntary open prehensor, the gripping digits in a voluntary close prehensor are closed upon an object to be grasped by actively exerting force on a control cable attached to the wearer's shoulder and neck using a harness. Voluntary close prehensors offer several important advantages over voluntary open prehensors. First, a voluntary close prehensor is more physiologically intuitive than a voluntary open prehensor. That is, a voluntary close prehensor requires a wearer to exert muscular force to grasp and hold an object while a voluntary open prehensor requires the wearer to relax his or her muscles to initiate and maintain a grip. Second, in a voluntary close prehensor, the gripping force applied to the object to be grasped by the wearer is directly related to the force the wearer exerts on the control cable. Requiring the wearer to exert force when grasping an object provides feedback to the wearer, thereby giving the wearer a sense of how strong his or her grip is upon the object. This feedback, also called physiological proprioception, allows the voluntary close prehensor to become an extension of the wearer's body with a natural feel and a confident grasp. Since voluntary open prehensors do not provide this feedback, the wearer is effectively removed from the gripping cycle. Third, by requiring that the wearer only exert the amount of energy necessary to attain the gripping force required to grasp an object, voluntary close prehensors conserve a large amount of the wearer's energy. In contrast, voluntary open prehensors require the wearer to stretch springs or rubber bands to separate or open the digits each time grip is to be applied to an object, regardless of the size or weight of the object. Any excess energy used to open the digits is wasted.
While voluntary close prehensors are generally more energy efficient that voluntary open prehensors, voluntary close prehensors still require the wearer to exert significant energy while maintaining a grasp on an object. Therefore, wearers desire voluntary close prehensors that reduce the energy needed to grasp an object as much as possible while providing feedback as to the force the wearer is exerting against the object. Many types of voluntary close prehensors are known in the prior art. For example, U.S. Pat. No. 4,225,983 issued to Radocy et al. and U.S. Pat. No. 4,332,038 issued to Freeland both disclose voluntary close prehensors. Radocy et al. focus their prehensor design towards achieving optimally configured gripping surfaces for the prehensor that can be inexpensively manufactured using stamped plate construction techniques. While Radocy et al. provide a locking pawl to assist the wearer in maintaining a grip on an object, unfortunately, Radocy et al. require that the wearer manually actuate the locking pawl. Freeland discloses an artificial hand with a pivotal thumb to adapt the hand for gripping different objects. Unfortunately, Freeland does not provide an energy efficient device capable of assisting a wearer in maintaining a grip on an object.
Despite the well developed state of the prior art, there remains a need for a voluntary close prehensor that conserves the energy expended by the wearer to size and grip an object. Preferably, the voluntary close prehensor will mechanically assist a wearer in maintaining the grip on the object without requiring any additional manual intervention by the wearer.